This document visualizes proposed policy scenarios for adding more green space to the City of Denver. These scenarios will be considered in a health-impact assessment.
Add native plants to all census block groups such that
Create native buffers around riparian areas (rivers, streams, lakes, and ponds) of the following sizes:
Initiatives related to green infrastructure and stormwater management. (Please refer to corresponding section for details.)
Replace a portion of parking-lot surface with native plants:
For all scenarios, we measured green-space exposure by the normalized difference vegetation index (NDVI) measured by the Landsat-8 satellite at a spatial resolution of 30 meters squared. To determine the target (alternative) NDVI value, we measured the NDVI of a 100% native zone in the Denver Botanic Gardens on several cloud-free spring and summer days. The NDVI value was about 0.5 in this area, so, preliminary, we consider 0.5 as the NDVI corresponding to native plants (subject to change; please see corresponding document).
Under the first scenario, which we view as the most ambitious, we consider the expected impact on mortality by applying a green-space intervention to all census block groups, without specifying where. This scenario was informed by conversations with colleagues who stated that
We first measured the mean NDVI of each census block group on July 4, 2021. The weighted block-group-level mean is presented here, weighted by the proportion each 30 square-meter pixel covers by the census block group. For example, if half of a pixel overlaps the block group, it receives a weight of 0.5 in the weighted average. We also removed bodies of water before measuring NDVI.
We excluded census block whose baseline NDVI was above the native-plant threshold:
We measured NDVI in a 200-foot buffer, a 100-foot buffer, and a 50-foot buffer around all bodies of water in Denver. We downloaded bodies of water from OpenStreetMap (code here). We defined residential exposure to these riparian areas as those individuals living within a 500-meter buffer, following research on green-space and health that have defined green-space exposure based on residential proximity. (Rojas-Rueda et al. 2019) We estimated the number of people in this buffer by multiplying the population density of the census block group by the intersecting area.
The below map depicts mean NDVI in the portions of census block groups that intersect a 200-foot buffer as well as those pieces that intersect the part of the 500 m buffer that would not be intervened upon, i.e., the part between the 200-foot buffer and the edge of the 500 m buffer.
We spoke with representatives at the local Office of Green Infrastructure, and they described three categories of initiatives–some planned, some aspirational–that could include native or adapted plants.
We were provided a list of planned projects throughout Denver. Based on our conversations, we assume that about 75% of the project’s footprint would consist of native or vegetation, so our intervention is defined as changing the NDVI values of these polygons from their baseline value (on July 4 2021) to the native-plant value of 0.5 As we did for the riparian areas, we defined exposure to the projects as those individuals living within a 500 m buffer of the projects. A map of baseline NDVI of the projects themselves and of a 500 m buffer around the projects is below.
to-do
The approach for estimating the health impact of the redevelopment controls is different from scenarios 2 and 3.a. because we do not know where the re-developments will occur. We thus simulated possible scenarios. As stated above, we anticipate the following number of parcels will be subject to these rules per year:
We gathered data on parcels from Denver’s Open Data Portal (Existing Landuse 2018) and measured their area. A subset of these parcels near Union Station is mapped below.
Then, from all parcels, we sampled 100 sites of size greater than 1 acre, 25 sites of size 0.5-1 acre, and 400 sites less than 0.5 acres. One such sample appears below.
From this point, we followed the same framework as we’ve done for other projects. We estimated that about 50% of this redevelopment would be native (subject to change), and we considered those exposed to these re-developments as anyone living within 500 m of them. The baseline NDVI of the sampled parcels and that of their corresponding 500m buffers is visualized below.
We obtained spatial data on parking lots from the City of Denver Open Data Catalog. About 9% of Denver’s area is a parking lot:
| area_mi2_prkng | den_area_mi2_total | prop_den_parking |
|---|---|---|
| 13.57 | 155 | 0.09 |
We similarly measured the NDVI on the parking lots as well as the NDVI in the census block groups that are within a 500-m buffer radius of any parking lots (which is most).
We present a small subset near Union Station, as the file size visualizing the NDVI measurements for the full city exceeds GitHub’s file-upload limits:
Copyright © 2022 Michael D. Garber